pgsql: Add new GUC categories corresponding to sections in docs, and

Log Message:
-----------
Add new GUC categories corresponding to sections in docs, and move
description for vacuum_defer_cleanup_age to the correct category.
Sections in postgresql.conf are also sorted in the same order with docs.

Per gripe by Fujii Masao, suggestion by Heikki Linnakangas, and patch by me.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.280 -> r1.281)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.280&r2=1.281)
pgsql/src/backend/utils/misc:
guc.c (r1.554 -> r1.555)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.554&r2=1.555)
postgresql.conf.sample (r1.287 -> r1.288)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.287&r2=1.288)
pgsql/src/include/utils:
guc_tables.h (r1.48 -> r1.49)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc_tables.h?r1=1.48&r2=1.49)